Are legacy phones a security risk?

[Tinov]

I'm a home user. I have a SIP-T18P, I'm not sure how old it is probably 10 years or more.

Is it safe to use on my home network? I don't care about encryption of calls or the like. I'm just worried that it might provide an entry point for a hacker.

Is there any up-to-date firmware available?

My network is thus:
The phone is attached to an unmanaged switch which also has a home PC (cable connection) a work-provided thin client (cable connection), and a wireless access point attached (mobile phones access the network via this). The switch is connected by cable to a home router which is a DHCP server. The phone is set to get IP address by DHCP. The network addresses in use are unrouteable (192.168.x.x). Service provider is Sipgate.

Any opinions or comment most welcome.